1. Field of the Invention
This invention relates generally to the recording and reproducing of color and momochrome or black-and-white video signals, and more particularly is directed to improvements in the automatic color control and color killer circuits of apparatus, such as, the so-called VTR, for reproducing such video signals.
2. Description of the Prior Art
In the recording of video signals on magnetic tape, for example, by means of a VTR, the tape is guided in a helical path about a portion of the periphery of a guide drum having rotary heads associated therewith so that, as the tape is advanced longitudinally, the heads successively record video signal information in a series of parallel record tracks which extend obliquly a cross the tape. The same apparatus may be used to reproduce or play back the previously recorded video signal information by causing the rotary heads to scan the successive oblique record tracks. Video signals are divided into field intervals, two of which comprise a frame, and line intervals, with each line interval including a blanking interval during which synchronizing signals are transmitted to control the scanning apparatus of a television receiver. Usually, when recording video signals by means of a VTR, a field interval is recorded in each of the oblique tracks on the magnetic tape. Color video or television signals, as transmitted, consist of a luminance or brightness signal and a chrominance component or signal comprised of modulated color subcarriers which, for example, in the case of the NTSC signal, have a standard frequency of approximately 3.58 MHz. Further, the band of the chrominance component contains burst signals occurring during the line blanking intervals and having the standard frequency of the color subcarrier. In such color video signals, as transmitted, the frequency band of the chrominance component is within that of the luminance signal.
When recording color video signals as described above, particularly for reproduction by means of a VTR intended for home use, it is well known to separate the luminance signal and the chrominance component, whereupon, the luminance signal frequency-modulates a suitable carrier to provide a frequency-modulated luminance signal, and the band of the chrominance component is shifted, that is, the chrominance component is frequency-converted, for example, from its standard carrier frequency of 3.58 MHz to a carrier frequency of 688 KHz., so as to occupy a frequency band below that of the frequency-modulated luminance signal. The frequency-converted chrominance component is then combined with the frequency-moldulated luminance signal to provide a composite signal which is recorded, as aforesaid. In reproducing the color video signal from such recorded composite signal, the frequency-modulated luminance signal and the frequency-converted chrominance component are separated from the reproduced composite signal, for example, by a high pass filter and a low pass filter, respectively, the separated frequency-modulated luminance signal is demodulated to derive the luminance signal, and the separated frequency-converted chrominance component is reconverted to its original or standard carrier frequency, for example, of 3.58 MHz, and is then combined with, or added to the luminance signal to constitute the reproduced color video signal. The amplitude or level of the chrominance component in the reproduced composite signal determines the color level of the reproduced color video signal. When the VTR has two or more rotary magnetic heads which successively scan the tape, as is usually the case, such heads may have different recording and/or reproducing characteristics with the result that the level or amplitude of the reproduced chrominance component will vary or change for successive field intervals. Further, the gains of recording and reproducing amplifiers of the VTR may be adjusted differently during the recording and reproducing, respectively, of the composite signal, particularly when the recording and reproducing operations are performed with different VTRs, with the result that the level of the reproduced chrominance component, and hence the color level of the reproduced color video signal, will not correspond to the color level of the original color video signal. Finally, due to the high speed with which each of the rotary magnetic heads scans the tape, the contact pressure of the head against the tape may vary for successive field intervals, and even during each field interval, to cause variations or irregularities in the level or amplitude of the reproduced chrominance component.
In order to avoid changes in the color level of the reproduced color video signal due to the above described variations or irregularities in the level of the reproduced chrominance component, existing VTRs are provided with an automatic color control (ACC) circuit which includes a variable gain amplifier for the reproduced chrominance component and a gain control circuit for such amplifier which adjusts the gain thereof so as to provide the reproduced chrominance component at its output with a stable level or amplitude. In the conventional ACC circuit for a VTR as described above, the burst signal is extracted from the reproduced chrominance component following the passage thereof through the variable gain amplifier, and either before or after its reconversion to the original or standard carrier frequency, and the level of the extracted burst signal is detected to provide the gain control signal for the variable gain amplifier through which the reproduced chrominance component has its level controlled. The detected level of the extracted burst signal is also used to control a color killer circuit included in the reproducing system of the VTR and by which the adding of the frequency-reconverted chrominance component to the luminance signal is halted when the detected level of the extracted burst signal is below a predetermined value and thereby indicates that the level of the reproduced chrominance component is insufficient to provide reliable color image reproduction.
However, upon recording a monochrome or black-and-white video signal with a VTR of the described type, it is very common to increase the band width of the lower side band of the frequency-modulated luminance signal which is recorded. Thus, in recording a monochrome video signal, the band of the frequency-modulated luminance signal which is recorded will extend, at its lower side, into the frequency band that would be occupied by the frequency-converted chrominance component during the recording of a color video signal. During the reproduction of a monochrome video signal recorded, as aforesaid, the circuit arrangement provided for detecting the level of the burst signal when reproducing a color video signal may erroneously detect the lower side band components of the luminance signal and thereby cause erroneous or misoperation of the color killer circuit.
In order to avoid such erroneous or misoperation of the color killer circuit, it has been proposed to interpose, in the circuit for detecting the burst signal, a crystal filter which is tuned to, or resonant at the frequency of the extracted burst signal so that only the existence of the latter, when reproducing a color video signal, will be detected. Although such crystal filter prevents misoperation of the color killer circuit when reproducing a monochrome video signal, the crystal filter gives rise to unstable operation of the ACC circuit when reproducing a color video signal. More specifically, the crystal filter has an extremely high Q or resonant factor so that ringing waves are generated at each interval when the burst signal is applied to the filter. The amplitudes of such ringing waves are not respresentative of the level of the burst signal, or of the reproduced chrominance component, and, therefore, the ACC circuit has a slow or sluggish response. Further, in a VTR, the reproduced composite signal is subject to time base errors, that is, the burst signal extracted from the band of the reproduced chrominance component does not always have precisely the standard carrier frequency. Since the crystal filter sharply detects any variation of the extracted burst signal frequency from the standard frequency at which the filter is resonant, such frequency variation of the extracted burst signal is falsely identified as a change or variation in the level of the reproduced chrominance component for which the ACC circuit seeks to compensate erroneously.